Dirt-Cheap 3-D Spatial Audio

With the addition of free audio software, an ordinary inexpensive surround sound card becomes the basis for a 3-D cube for simulation, visualization or gaming.

System Validation

After all of the testing and calibration was completed, we performed
two informal, qualitative user tests that would help us validate our
new low-cost spatial audio system. The first test evaluated how the
new sound system configuration with eight speakers compared with our
previous planar configuration containing four speakers. The prior
configuration simply used the four speakers on the top of the cube
array. We realize that directly comparing these two configurations is
somewhat biased, due to the placement of the four-speaker array being
located above the user's head. It would be fairer to compare against a
four-speaker array located at the height of the user. However, by using
the top four speakers, we were able to switch between the two
configurations without dismantling our installation.

We performed the experiment by asking a few test subjects to stand in
the middle of the immersive room and listen to sounds played for each
configuration. We played different sequences of audio on both speaker
configurations and made use of the full range of speakers available.
The subjects were not told which configurations were being used, nor
in which order the pairs of configurations were presented. Several
iterations of the pairs of configurations were tried for each subject.
After each pair was presented, the subjects rated the two systems.
Admittedly, this was not a scientific test, as is evidenced by several
unaddressed biases, but all test subjects clearly preferred the
eight-speaker configuration.

The second user test evaluated how well the listener is able localize
the source of the audio using the eight-speaker configuration. Again, the
subjects were tested and each were asked to stand in the center of the
immersive room. Each subject was presented with several sounds played
one at a time and originating from different positions surrounding the
subject. The subjects were asked to point in the direction of the
sound source, as they heard it. The visual system was not running, so
the users did not get visual cues as to the sound source's location.
The subjects were able to localize the sounds with a high degree of
accuracy, especially with respect to elevation.

The implementation of our 3-D spatial audio system integrated with our
immersive room really enhanced the simulation and training demos we
have. Our completed system has improved dramatically the sense of
immersion when running the demos. A simulation user easily perceives
helicopters and jets flying overhead and a tank rumbling down one of
the many streets nearby in the virtual world. The perception of depth
from the source of audio is conveyed accurately and also includes
doppler effects. Our system is a step above a four-speaker solution we
had previously using the Microsoft DirectSound API. It also is a good
replacement for the capable but outdated and unsupported eight-speaker solution we had
running using another expensive hardware and software platform.

Conclusions and Future Work

We have devised a true 3-D spatial audio solution that is low cost and
has comparable quality to expensive high-end commercial systems. The
3-D spatial audio solution allows sound effects to be generated from
all directions surrounding a user, not only planar directions. We
accomplished this feat by using only commodity hardware and open-source
software. We feel this feature, now available at an affordable
price, creates numerous options for game and virtual reality
system developers.

We feel our system leads the way for others to devise similar
solutions with current and future commodity audio equipment. The
developer needs only to purchase a Dolby Surround Sound
7.1 audio card, four pairs of low-cost speakers and audio cables. We
spent less than $150 US on hardware—Audigy 2 audio card and audio
cables—as we already had speakers available.
From start to
finish, including hardware and software debugging, configuring and
testing, we spent less than a month developing the low-cost 3-D spatial
audio system. We feel that using this document as a guide, it should
be possible for others to implement this system in less than a week.

Future Enhancements

Although the system currently meets our needs quite nicely, these
features would be nice to add to the 3-D spatial sound API in the
future:

Directional sound cones: directional sound cones are a
mechanism to provide directional sound with the strongest intensities
propagated along the central axis of the cone and weakest toward the
edges. Because many sound sources are directional by nature, such as sound
emanating from a megaphone, directional sound cones would allow
these sound sources to be generated more accurately. Also, because some
major APIs, such as Microsoft DirectSound, offer sound cones, it would be nice
to offer such a feature.

Additional environmental reverberation effects: although
a number of simple environmental reverberation effects are
available in the common 3-D spatial audio APIs, supporting more
sophisticated effects, such as sound reflection and absorption off of
different surfaces, greatly would enhance the listening experience.
This is an area of ongoing research, and the Mustajuuri system would
be a good testbed for trying out new techniques.

Enhanced sound attenuation level: the current distance
attenuation models for sound in Mustajuuri are quadratic by nature,
thus they are fairly simple. In the real world, sound attenuation is much
more sophisticated and depends on heat, humidity, sound frequency and
many other factors. For example, low-frequency sounds generally
carry much farther than high frequency. Accounting for these
complexities could help significantly in providing distance cues.